Homosexuality has been an acknowledged aspect of human society since our earliest recorded history. In some civilizations, such as ancient Greece, homosexuality was relatively common and accepted. It is also a behavior that is not specific to humans. It has been documented in over 500 non-human animals, including penguins, bonobos, and grizzly bears.

Over the past few decades, evidence has begun to accumulate that homosexuality is a behavior that appears primarily due to biological or genetic influences. While environmental factors may play a part in the expression of a homosexual phenotype, most scientists would suggest their influence is not powerful enough to cause an otherwise heterosexual organism to become homosexual (although there are environmental conditions that may encourage transient homosexual behavior, e.g. captivity).

This dependence on biological factors, however, creates a paradox for evolutionary theorists. The assumed goal of all organisms, and thus of evolution, is reproduction—the passing on of one’s genes to a new generation. Since homosexuals reproduce at a much lower rate than the heterosexual population, one might think a genetic basis for homosexuality—even one that involved several different genes—would by now have disappeared from the gene pool.

A number of hypotheses have been proposed to explain this paradox, although none of them has gained the full support of the scientific community. One early explanation, which has for the most part fallen out of favor, is kin selection. Kin selection occurs when an organism acts in a way that fosters the reproductive success of its relatives, even at a cost to its own reproductive success. In this scenario, childless homosexuals might put more effort into helping raise nieces or nephews. These relatives might carry some of the same genes as the homosexual, and thus if they eventually reproduce they may pass on genes essential for homosexuality. Evidence in support of this theory is limited, however, and most feel it doesn’t tell the complete story.

More accepted explanations today include: 1) overdominance, 2) maternal effects, and 3) sexually antagonistic selection. Overdominance occurs when a heterozygous version of a gene provides an organism with some type of reproductive advantage. For example, a straight man might have a heterozygous gene that, if it were homozygous, would increase his chances of being gay. In a heterozygous state, however, it could result in increased sperm motility. Thus, when it is passed on through reproduction, its recessive allele (which predisposes for homosexuality) is as well.

The maternal effects hypothesis suggests a fetus is influenced by the environment of the mother’s womb, resulting in changes that predispose one toward homosexuality. This hypothesis was proposed after evidence began to appear that homosexuality in males is predicted by high numbers of older brothers. In trying to make sense of this statistic, scientists postulated that a mother might build immunity to male-specific antigens with each birth of a male child. The progressive immunization to these male antigens may eventually affect the brain of a male fetus. This could happen, for example, if antibodies crossed the placenta and attacked male-specific regions of the brain necessary for sexual differentiation.

Sexually antagonistic selection, which appears to have the most evidential support at this point, is a mechanism whereby genes are spread throughout a population by giving a reproductive advantage to one sex while disadvantaging another. In the case of homosexuality, the mother may have increased fertility at the expense of her child’s ability to procreate. This concept can be expanded upon to include female homosexuality as well, whereas any trait with a gender-specific benefit may have evolved by increasing female fertility.

Although the evolutionary explanation of homosexuality is still elusive, evidence for a biological basis for homosexuality continues to accrue. An article published this week in the Proceedings of the National Academy of Sciencesreports that the brains of homosexuals have similarities to the brains of heterosexuals of the opposite sex. Using magnetic resonance imaging (MRI), researchers compared heterosexual and homosexual brains. They found that the brains of heterosexual men and homosexual women have a slightly larger right hemisphere than the brains of gay men and straight women. It has been found in the past that there are differences in activity between the right and left hemispheres in different sexes.

The researchers in this study also found, using positron emission tomography (PET), that the connectivity of the amygdala, an area of the brain important in emotion, was more similar in lesbians and straight men, and gay men and straight women, respectively.

These findings provide further evidence for homosexuality having a biological origin. Of course they don’t preclude the possibility that there are environmental influences on the expression of homosexuality. Instead, however, they make much less plausible the often bandied about argument that homosexuality is a “choice”. As the correlation between brain structure and a particular behavior becomes stronger, the involvement of choice usually decreases proportionally.

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